The Sex-lethal amino terminus mediates cooperative interactions in RNA binding and is essential for splicing regulation.

Sex-lethal (Sxl) acts as a binary switch that regulates Drosophila sexual differentiation and dosage compensation and also maintains a stable female state through autoregulation. As part of a cascade of genes that are regulated by sex-specific splicing, Sxl controls the sex-specific splicing of transformer (tra) RNA and also its own RNA. Sxl contains two RNP-CS (RNA-binding) domains and is known to bind tra pre-mRNA near the alternative 3' splice site, thus blocking use of that site to give the female-specific splicing pattern. Here, we test how Sxl protein interacts with Sxl RNA during autoregulation. We show that Sxl not only binds Sxl pre-mRNA near the alternative 3' splice site but also at distant, multiple sites surrounding the Sxl alternative exon. Moreover, Sxl binds cooperatively at these multiple sites. The Sxl amino terminus is essential for the cooperative interaction and is also required for regulatory activity in vivo. It appears that this region of Sxl protein, which resembles regions in some other RNA-binding proteins, is a domain that mediates protein-protein interactions during RNA binding and plays an important role in splicing regulation.